It is well known that circuit packaging can have profound effects on the properties of integrated circuits (ICs). Lowering the dielectric constant (K) of the substrate materials in ICs increases circuit speed, reduces power consumption and also minimizes electronic cross-talk, thus allowing for higher functional density. Polymeric materials typically offer lower dielectric constants than ceramic materials, but for many applications the reliability and superior thermal conductivity of ceramic packages make them more desirable. The low-K ceramic dielectrics currently used are predominantly filled glass systems in which the glass is typically a borosilicate composition such as that disclosed in JP 62,138,357 or an aluminosilicate such as that disclosed in JP 62,150,856. Systems of this type have caused some reliability and/or yield problems because of phenomena such as conductor blistering, staining and conductor "swimming" (spreading across the surface of the part). Dimensional control during re-firing is also a frequent concern. In addition, the inclusion of alkali ions, commonly used to lower glass viscosity, also produces glasses having significant ionic conductivity and can lead to electrolytic reactions and/or high dielectric losses.
One approach to reducing the reactivity of the dielectric systems is the use of crystallizable glasses. These materials, if correctly designed, will first sinter to a hermetic body and then crystallize during the firing process. During subsequent firings to complete the processing of the circuit, the mostly crystalline substrate is chemically and mechanically more stable than conventional filled glass substrates. Crystalline materials also typically have higher thermal conductivity and higher mechanical strengths than amorphous materials. Desirable properties for candidate crystallizable glass-based systems would include low dielectric constants and losses, coefficients of thermal expansion (TCE) matched to 96% alumina (6.2 ppm/.degree. C.) or silicon (3.5 ppm/.degree. C.), hermetic sintering and crystallization upon firing to 850.degree.-900.degree. C. to facilitate their use with precious metal conductors. The TCE of the dielectric system can be matched to alumina or silicon by use of appropriate fillers such as cordierite (to lower TCE) or AlPO.sub.4 (to raise TCE).